1. Field of the Invention
The invention relates to internal combustion engines, and in particular, to homogenous charge compression ignition (HCCI) engines.
2. Description of the Related Art
Emission control standards for internal combustion engines have tended to become more stringent over time. The sorts of emissions to be controlled tend to fall into at least four broad categories: unburned hydrocarbons, carbon monoxide, particulates, and oxides of nitrogen (NOx).
Unburned hydrocarbons and carbon monoxide tend to be products of incomplete combustion of a hydrocarbon fuel. Each atom of carbon in the fuel requires two atoms of oxygen with which to combine for complete combustion. If each carbon atom finds two oxygen atoms with which to combine, carbon dioxide is formed. The remaining hydrogen atoms combine with two oxygen atoms apiece to form water.
If only one atom of oxygen is available to combine with a carbon atom, on the other hand, carbon monoxide is formed. If no oxygen is available, hydrocarbons are left unburned. Thus, reduction of unburned hydrocarbons and carbon monoxide depends on the provision of adequate oxygen and temperature during combustion to oxidize the carbon atoms completely.
Compression ignition engines are generally run with an excess of air over the stoichiometric ratio to ensure adequate oxygen supplies are available for combustion. Particulates tend to be produced by reactions that are close to stoichiometric as well, so the availability of an excess of oxygen over stoichiometric may reduce those as well.
Nitrogen is a major component of air. Nitrogen is inert at standard temperature and pressure. Nitrogen becomes reactive, however, at heightened temperatures and pressures. The heat associated with high temperatures thus serves as a catalyst for nitrogen. High temperatures tend to be associated with complete combustion, since combustion is exothermic. Nitrogen will thus react with oxygen at the high temperatures associated with complete combustion, forming oxides of nitrogen.
One way to control the production of oxides of nitrogen is to limit the combustion chamber temperatures reached during combustion. Since heat associated with high combustion temperatures serves as a catalyst for nitrogen, reducing the peak combustion chamber temperature may reduce the reactivity of nitrogen. Since reducing the peak temperature ameliorates one of the conditions necessary for the production of oxides of nitrogen, there may be a consequent reduction in the quantity of oxides of nitrogen that are produced.
Fuel is injected, on the average, into the center of a combustion chamber in a conventional compression-ignition engine. The fuel is injected after the incoming air charge has been compressed sufficiently to ignite the fuel, and thus the fuel burns almost immediately. Since the fuel burns almost immediately, it has relatively little time to distribute itself evenly about the combustion chamber. Since the fuel is not distributed evenly during combustion, but rather is localized, a large quantity of fuel is available in a small volume to support combustion. Since a large quantity of fuel is available to support combustion, combustion proceeds for a relatively long period of time, and high temperatures of combustion are able to develop.
With HCCI engines, on the other hand, fuel is injected during the intake stroke or the compression stroke. The fuel thus has some time to propagate throughout the volume of the combustion chamber before combustion takes place. The combustion event occurs once the air charge has been compressed enough to raise its temperature to the kindling temperature of the fuel. Furthermore, the swirling and tumbling of the air charge during intake or compression may promote distribution of the fuel before the combustion event takes place.
Since the fuel has time to propagate throughout the combustion chamber volume before ignition takes place, ignition may occur simultaneously throughout the combustion chamber volume. This may, for example, allow the combustion process to rely less on propagation of a flame front to burn the fuel than would be the case with conventional compression ignition.
The combustion rate may also be higher, since there will be no delay associated with waiting for a flame front to progress across the combustion chamber. This may allow a more dilute mixture of air and fuel to be used. This may also allow the peak temperature to be reduced, thereby reducing formation of oxides of nitrogen, since the fuel burns completely in less time than it would take for comparable localized combustion. This may allow the combustion process to take place at lower temperatures than it would were the fuel more concentrated.
Since most of the expansion in an HCCI engine occurs once there is sufficient fuel available to auto-ignite and the air charge has been compressed enough to raise its temperature to the kindling temperature of the fuel, the timing of the expansion event may vary somewhat from cycle to cycle. The timing of the expansion event in an HCCI engine may thus be relatively more difficult to control than the timing of the expansion event in a conventional compression ignition engine. Variability of expansion timing may manifest itself as roughness or pre-detonation, also known as “knocking.”
Combustion may proceed in stages. Combustion of diesel fuel, for example, may be characterized by different reactions at different temperatures. The temperatures associated with combustion may vary over time. Since the character of a reaction may depend on the temperature at which it occurs, the reactions may vary over time as well.
Many of the constituents of diesel fuel exhibit a molecular structure similar to n-heptane (C7H16). The chemical kinetics of oxidation of n-heptane may start with hydrogen being extracted from C7H16 as shown below:C7H16+O2=C7H15*+HO2*C7H16+OH*=C7H15*+H2OC7H16+HO2*=C7H15*+H2O2 
There are two possible reactions for C7H15* and O2. The first reaction, known as “cool flame combustion”, occurs at temperatures below about 1000 K. This reaction may be characterized as C7H15*+O2=C7H15OO*. Cool flame combustion dominates the combustion process while temperatures remain below about 1000 K, and produces C7H15OO*.
This reaction may be followed by an isomerization reaction in which C7H15OO* is converted to *C7H14OOH, or: C7H15OO*=*C7H14OOH. If an oxygen molecule is added to *C7H14OOH, an oxohydroperoxide radical (OOC7H14OOH) may be produced. The oxohydroperoxide radical may then isomerize further and decompose into a relatively stable ketohydroperoxide species and OH*.
Thus, numerous OH* radicals may be formed during cool flame combustion. A significant amount of carbon monoxide (CO) may be formed during cool flame combustion as well. In addition, water (H2O) forms as a result of C7H16 reacting with OH*, or C7H16+OH*=C7H15*+H2O. This reaction is highly exothermic and proceeds quickly.
The second reaction of C7H15* and O2 results from “auto-ignition”, and takes place at temperatures above about 1000 K. This reaction may be characterized as C7H15*+O2=C7H14+HO2*. The temperature of the mixture may rise as hydrogen is extracted from the n-heptane. If the temperature rises above about 1000 K, H2O2 may decompose into two hydroxyl radical OH* molecules via a chain branching step: H2O2+M=OH*+OH*+M.
One of the factors that determines whether combustion temperatures rise above 1000 K is the availability of fuel. Cool flame combustion may be maintained indefinitely in the absence of sufficient fuel to support auto-ignition, other things being equal. If, however, fuel sufficient to support auto-ignition becomes available after the onset of cool flame combustion, auto-ignition may commence, and temperatures may rise above 1000 K, possibly reaching 1400 K to 1500K. Combustion at temperatures above 1000 K may be more complete, and further extraction of hydrogen from C7H16 may result in the conversion of the remaining OH* radicals to H2O, and CO to CO2. The higher temperatures, however, may result in more oxidation of nitrogen as well.
If, however, some cool flame combustion has taken place beforehand, the presence of leftover carbon dioxide (CO2) and water (H2O) may quench combustion somewhat in the manner of internal exhaust gas recirculation (EGR). Internal EGR may thus lower flame temperatures and limit production of NOx just like conventional EGR once auto-ignition has taken place.
Furthermore, excess OH* radicals present during auto-ignition may result in quick production of C7H15* and its decomposition at higher temperatures. Finally, hydrogen may be extracted from a significant amount of the fuel injected during the main injection event having due to the presence of OH radicals, resulting in lower soot formation.